Patch antennas are the work-horse antenna for receipt of L-band signals broadcast from satellites. These signals include Global Navigation Satellite Systems (GNSS) and other communications systems such as Globalstar, Iridium and a host of other L-Band satellite communications systems such as Inmarsat.
The civilian signals transmitted from GNSS satellites are right hand circularly polarized (RHCP). Circularly Polarized (CP) signals have the advantages that the received signal level is independent of the rotation of a CP receiving antenna in a plane orthogonal to the propagation vector.
Conceptually, circularly polarized signals can be thought of as comprised of two orthogonal, linearly polarized signals offset in phase by 90 degrees (“in phase quadrature”), as shown in FIG. 1.
When a circularly polarized wave is reflected at a low impedance surface (such as metallized glass), the polarization direction becomes reversed or “cross polarized”, so that a RHCP wave becomes LHCP and vice versa. Multipath interference can cause pure CP waves to become instantaneously elliptical (i.e. tending toward linear polarization) when the ‘direct’, RHCP wave is combined with a ‘reflected’ LHCP wave.
A receiving antenna with a “pure” CP response has the property that cross polarised signals are strongly rejected (−20 dB or better), significantly reducing the response to reflected signals, while reception of the direct signal is unaffected.
Considerably better positioning accuracy can be obtained in GNSS systems that have antennas with a “pure” CP response.
It has been shown that GNSS receivers with the capability to track satellites from more than one constellation are able to offer considerably improved positioning, primarily because of the larger number of satellites that can be simultaneously tracked (“in view”).
As a consequence almost all new GNSS receiver chips now in development, as at the date of this application, are designed to receive signals from multiple constellations.
While all GNSS constellations broadcast navigation signals on multiple frequencies, this disclosure is concerned primarily with those broadcast in the “L” band. The GNSS constellations in service, or planned, are as set out below:                U.S: GPS-L1: 1575.42 MHz (in service)        Russian Federation: GLONASS-L1: 1602 MHz (+13, −7)*0.5625 MHz (in service)        People's Republic of China: COMPASS-L1: 1561 MHz (being deployed).        Europe: Galileo L1: 1575.42 MHz (overlay on the US GPS frequencies, planned).        
Patch Antenna Types
The most widely used antenna element for reception of GPS L1 signals has been single feed ceramic patches, see FIG. 2.
Typically, such antennas are comprised of a rectangular block of low loss, high dielectric substrate material (1) such as ceramic, typically 25 mm×25 mm×4 mm or smaller. A first major surface is metalized as a ground plane (2), and a resonant metal plate is metalized on the second major surface (3). The feed pin (4) is connected to the resonant metal plate and isolated from ground, passing through an aperture in the ground plane.
This structure constitutes two orthogonal high-Q resonant cavities, one along a first major axis (5) and another along the second major axis (6) of the patch.
There are a number of well-known techniques commonly used to elicit a CP response from a single feed patch element. Two widely utilized techniques are shown in FIG. 3(a) and FIG. 3(b), wherein the feed pin (12(a)) and (12(b)) is connected to a resonant plate (10(a)) and (10(b)), having corner chamfers (9) and/or small dimensional offsets, each associated with specific feed pin locations (7), (8).
The patch is electromagnetically coupled to free space by the fringing fields between the resonant metal plate (10) and ground plane (11).
Small single feed antennas with this structure are characterized by low cost, narrow bandwidth, and a “pure” CP response at a single frequency.
Such antennas are ideal for low cost GPS receivers because the GPS L1 signal is a single frequency carrier, direct sequence modulated with the navigation and spreading signals.
The nature of a circular E-M wave inherently suggests that a circularly polarized antenna can be realized with two linearly polarized antennas that are disposed orthogonally, with summing means to combine the signals present on the two feed pins in phase quadrature.
Such a structure is achieved with a dual feed patch antenna (see FIG. 4). This, more general architecture also utilizes a substrate (12) with a ground plane (13) on first side and a square resonant metal plate on the second side (14), but has two feed pins (15) (16), connected to the resonant metal plate, each isolated from the metallized ground plane. The feed pins are equally offset from the patch center and located so that the angle subtended between two lines drawn from each feed pin location to the patch centre is 90 degrees.
Typically, but not necessarily, the feed pin positions are located on the major ‘X’ axis (17) and ‘Y’ axes (18) in the plane of the patch.
In this configuration the antenna provides two orthogonal linear antennas. At all frequencies, there is a high degree of electrical isolation between the two feed pins.
If the signals which are in phase quadrature and which are present at the feed pins, are combined in phase quadrature, the response of the antenna will either be LHCP or RHCP depending upon the polarity of the phase offset of the Q (quadrature) signal phase relative to that of the I (In-phase) signal.
Two alternate combining networks are shown in FIG. 5(a) and FIG. 5(b). With reference to FIG. 5(a) the function of a combining network can most readily be understood in terms of summing device (19), with isolated ports (such as a Wilkinson combiner), having a 90 degree phase shift in one branch (20) (such as a λ/4 transmission line), connected between a first antenna feed (21) and a first input (22) of the summing network (19), with the second antenna feed (23) connected directly to the second input to a summing device. FIG. 5(b) shows another form of quadrature combining network that utilizes a 90 degree hybrid, a device that has precisely the required transfer function.
Dual feed antennas (including variants with aperture coupled feeds) are characterized by a narrow bandwidth, but have a “mathematically correct” response. This provides a “pure” CP response over the entire bandwidth of the antenna. The requirement for a hybrid combiner makes the dual feed architecture somewhat more costly than single feed.
Relative Characteristics of Patch Antennas
The axial ratio (“AR”) parameter for a CP antenna is a measure of the maximum to minimum response to a linearly polarized wave propagating in a plane orthogonal to a line to the antenna center.
The frequency response of a single feed patch to linearly polarized excitation is a function of the field rotation relative to the receiving antenna. This effectively reveals the axial ratio. In FIG. 6, curves A and B show that the axial ratio for a typical 25 mm×25 mm×4 mm single feed patch at GPS and GLONASS frequencies is about 8 dB for certain rotation angles of the linear field (shown at Zenith).
This shows that, by its nature, a single feed patch element exhibits a truly circular response (AR=0 dB) only where the curves for all rotation angles intercept, i.e. at a single frequency. The corollary is that at the 1 dB bandwidth corner frequencies, the response is strongly elliptical.
Well-tuned single feed patch antennas are ideal for GPS because GPS L1 navigation signals are DSS modulated single frequency carriers. However reception of multiple constellation signals requires antennas to operate over an extended bandwidth.
In urban regions GNSS signals are commonly reflected from buildings so that a delayed, cross-polarized signal is superimposed on the direct signal. The effect of poor axial ratio in a receiving antenna is that the cross polarized signals are not strongly rejected by the antenna so that the signals input to a GNSS receiver are “smeared”. They are also subject to “flutter” for individual satellite signals due to cross-polarization interference (standing wave) effects.
Dual feed patch antennas theoretically can exhibit a virtually ideal axial ratio (AR=0 dB) over the entire bandwidth of the patch. This is because each axis is isolated from the other and, at higher elevation angles, both receive equal amplitudes for an incident CP wave, and contribute equally. Thus, dual feed antennas offer considerably improved performance for multi-constellation reception.
The feed impedance of a single feed patch (See FIG. 2) is a strong function of the offset distance of the feed pin from the patch center. At the resonant frequency, with the feed pin at dead centre of the patch, the feed impedance is a short circuit to ground, and a high impedance with the feed pin offset close to the edge the resonant metal patch.
For a 4 mm×25 mm×25 mm patch, the feed impedance is approximately 50 Ohms with the feed pin offset by approximately 2 mm from the patch centre. To minimise feed inductance, the physical feed pin diameter is typically about 1.5 mm diameter. Thus, the dimensions in a small patch element are too small to accommodate dual feed pins with a convenient feed impedance.
Given sufficient radio frequency (“RF”) gain, the limitation to sensitivity of a GNSS receiver comprised of an antenna and a receiving circuit, is the ratio of the received signal carrier power to the total system noise, commonly referred to the antenna terminals, in a one Hertz bandwidth (“C/No”)
Total system noise is at least the sum of galactic noise, local black-body radiation, man-made noise, noise generated in the receiver, plus effective noise generated as a function of losses in the antenna.
As is known, it is important to provide an optimum noise match (impedance) between the antenna and the first RF amplifier stage (known as “Γopt”). Thus it is also important that the feed impedance of the antenna have a value that is an optimum noise match to the first RF amplifier stage, requiring a minimum of additional matching components.
Small single feed patch elements can be configured to provide a convenient (50 Ohm) real impedance but only at a single frequency.
From the aforesaid, it will be appreciated that single feed antennas are considerably deficient for reception of multiple constellation GNSS signals and it is not feasible to realized a more appropriate dual feed patch according to prior art on a small high dielectric substrate.
Furthermore, the dual feed antenna has a requirement for a signal combining network. All known combining network are relatively large compared with the dimensions of a miniaturized antenna, and all represent additional cost. There is therefore a need for a means to achieve the same combining function using smaller less expensive components.